1. Field Of The Invention
This invention relates to a thermally controlled persistent current switch for high energy superconductive magnets, and more particularly to such a switch employing a cupro-nickel matrix superconductor.
2. Discussion Of The Prior Art
Switches for turning superconductive magnet circuits on and off are well known. They typically comprise a length of superconductive wire and a heater element. The switch is cooled to a temperature well below the critical temperature of the superconductor (e.g., 9.degree. Kelvin) by immersing it in a cryogen such as liquid helium. At or below the critical temperature, the resistance of the wire falls to zero, thereby making the wire superconductive. In the superconductive or persistent state, the switch is "on" because it has no resistance. The switch is turned off by turning the heater element on, which raises the temperature of the superconductive wire above the critical temperature, thereby producing a finite resistance to the flow of current.
The coils of superconductive switches are made up of as many turns of superconductive wire as are necessary to achieve the desired resistance in the "off" state and provide a great enough heat capacitance in the switch to avoid damage when the switch is off. In prior switches which have employed copper matrix superconductor, a very long wire has been required to achieve even a relatively small resistance. Because of the small resistance, the switch also had to be able to absorb large amounts of energy. For example, in one such switch, 2200 feet of copper matrix superconductor was required to achieve a resistance of approximately 0.05 .OMEGA. at 10.degree.-20.degree. K. In the application of a superconducting switch to the charging circuit of a superconducting magnet, a higher resistance is desirable because it allows less energy absorption by the switch when it is off, thereby reducing the boil-off of the cryogen during charging and discharging of the magnet.
Another consideration in superconductive switches is current carrying capacity. High current carrying capacity (e g. 500 Amperes or more) when "on" is desirable and required in some applications. Copper matrix switches have generally been capable of high current densities, but at the expense of the relatively poor "off" characteristics referred to above.
Cupro-nickel matrix superconductors are known, but they have not been successfully employed in high current superconductive switches. Such superconductors are inherently extremely unstable. Slight movements in a magnetic field can cause them to "quench", losing their ability to conduct at zero resistance.